1. Field of the Invention
The disclosures herein generally relate to an image forming apparatus such as a digital color copy machine or a printer, especially, a technology for correcting density that can reduce a differentiation of printing density caused by change over time.
2. Description of the Related Art
To reduce a differentiation of printing density caused by change over time which appears on a printed image output by an image forming apparatus, there is a known method in which a detecting pattern is formed on an image bearing member, which is captured as a detected value by a sensor, which is then used together with a target value for correction to calculate a corrected value to be reflected in a density conversion process in a later stage. However, in the conventional method, since a target value for correction is set to a fixed value, there is a problem in that a large difference of density may be perceptible by appearances between images before and after the correction process.
For example, Japanese Patent No. 4297168 discloses a technology for correcting density of an image based on a relationship between a measured density of dither data and a target density of the dither data set in advance, to prevent an occurrence of a density measurement error caused by change over time or an occurrence of a density measurement error caused by lot variations. However, the problem of a large difference of density perceptible by appearances between images before and after the correction process cannot be solved because a target value for correction is set to a fixed value.